Methods and devices for non-invasively determining the percentage of hemoglobin which is carrying oxygen are generally known in the medical field. This percentage is referred to as hemoglobin saturation. The type of hemoglobin which carries oxygen is called oxyhemoglobin, while the type of hemoglobin which is devoid of oxygen is called deoxyhemoglobin. Hemoglobin saturation is of interest since it indicates the degree of oxygenation of the blood in the tissues.
A device which can perform a non-invasive measurement of hemoglobin saturation is generally referred to as a pulse oximeter. With this device, light is transmitted through a monitoring site which is usually the finger, ear or toe. The pulse oximeter measures absorbances in the visible and near-infrared ranges of the electromagnetic spectrum, in order to measure hemoglobin saturation. It is well known that oxyhemoglobin is redder than deoxyhemoglobin. As such, deoxyhemoglobin nominally absorbs light at 603 nm more intensely than oxyhemoglobin. There is another difference in absorption characteristics of these two species which is not visible: oxyhemoglobin nominally absorbs light at 940 nm more intensely than deoxyhemoglobin. The quantity of light absorbed at these two wavelengths is characteristic of a particular mix of oxy and deoxyhemoglobin. Hemoglobin saturation is calculated using absorbance data and a prediction curve which is generated by a large population study which correlates pulse oximetric data with traditional hemoglobin saturation measurements.
Tissue contains absorbing substances other than the species of hemoglobins. However, generally a pulse oximeter can isolate the absorbances of the hemoglobin species of interest from the absorbances of potentially interfering species. It does so by determining the difference between the absorbance of light by tissue before an arterial pulse and the absorbance of light by tissue at the peak of an arterial pulse. The difference in absorbance is attributed to arterial blood at the site of the measurement. In summary, the absorbance prior to a pulse is subtracted from the absorbance at the peak of a pulse to determine the percentage oxygen saturation of arterial blood hemoglobin.
U.S. Pat. No. 4,819,752, the disclosure of which is incorporated herein by reference, discloses a pulse oximeter type device which measures hemoglobin saturation using these principles. The device disclosed in this patent differs from prior an methods in the way in which it processes signals, in relation to isolating the pulsatile component, determining the size of the pulsatile component and in determining the size of the non-pulsatile component.
Similarly, U.S. Pat. No. 4,805,623, the disclosure of which is incorporated herein by reference, describes a spectrophotometric method of measuring the concentration of a dilute component such as hemoglobin in a light- or other radiation scattering environment. The disclosed crux of the invention involves simultaneous measurement of the absorbed/reflected light of the dilute and of the reference components. Essential features of the method employed in U.S. Pat. No. 4,805,623 include, determination of path-length and an extinction coefficient of the analyte in the light-scattering environment, along with use of complex theoretical formulas.
As is generally known to those of ordinary skill in the art, in-vivo spectrophotometric measurements are complicated by scattering losses, difficulties in path-length measurement and spectroscopic interference from species other than those of interest. Spectrophotometric analysis is typically based on a model that assumes pure collimated light is reduced in intensity only by absorbing species. The intensity is reduced by an exponential process known as "Beer's law", wherein absorbance is proportional to concentration. A classical Beer's law approach to analyte measurement in tissue, using a path-length and extinction coefficient determination however, generally gives clinically unacceptable results due at least in part to the complications referred to above.
The method of the present invention differs fundamentally from that of U.S. Pat. No. 4,805,623 ("the '623 patent") in that it does not require a light path-length or an extinction coefficient determination. Furthermore, in a preferred embodiment the method of the present invention employs a pulse based measurement by using a pulse oximeter which has been modified to make measurements at the appropriate wavelengths for hemoglobin concentration.
Spectrophotometric methods have also been utilized to measure hemoglobin concentration in-vitro. These methods are generally referred to using the terminology "in-vitro hemoglobinometry". In the most commonly used method of in-vitro hemoglobinometry, a blood sample is diluted, lysed and treated with potassium cyanide. An absorbance reading is taken at 540 nm and compared with that of a standard solution. This method is described in Clinical Diagnosis & Management By Laboratory Methods, Henry, John B. (W. B. Saunders Company, Philadelphia, 181h Ed. 1991).